1. Field of the Invention
The present invention is directed to a light source apparatus used in an optical scanning apparatus which is mounted on an image forming apparatus, such as a digital copying machine or a laser printer, and in particular to a light source apparatus having multiple light emitting devices.
2. Description of the Related Art
It is conventionally known that changes in temperature of an optical scanning apparatus cause changes in wavelength of its light source, positional displacement due to expansion and contraction of optical components, and changes in the refractive index or the like, thereby causing a change in an image location (at which an image is formed), on a surface to be scanned, in the optical axis direction of the scanning light.
For example, Patent Document 1 proposes a technology that adjusts linear expansion coefficients of members supporting the light source and the collimator lens in such a manner that the distance between the light source and the collimator lens does not change, thereby preventing change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light.
Patent Document 2 devises the lens structure of the collimator lens, whereby even if the wavelength of the light source changes, it is possible to prevent change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light.
Patent Document 3 proposes to provide a ring-shaped member on the tube of the collimator lens in order to hermetically seal the space between the collimator lens and the light source. In this case, however, the collimator lens is fixed based on the position of the ring-shaped member, and therefore, the ring-shaped member does not shift in the optical axis direction even if the ambient temperature changes. Accordingly, the technology of Patent Document 3 leaves the problem that a change in wavelength of the light source due to environmental change causes change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light.
If the tube of the collimator lens is made long, the position of the collimator lens can be shifted in relation to that of the ring-shaped member according to expansion and contraction of the tube. However, in this case, an end face of the tube away from the ring-shaped member is not fixed in the directions perpendicular to the optical axis, and therefore, the collimator lens may shift in a perpendicular direction. This sometimes causes the problem that the emitted light beam changes in a direction perpendicular to the optical axis.
Patent Document 5 proposes a technology that provides a spring means and a screw means in order to adjust the position of the collimator lens in the optical axis direction and further provides another spring means in the direction perpendicular to the optical axis. Accordingly, it is possible to prevent the positional displacement of the collimator lens in the perpendicular directions during the adjustment of the collimator lens in the optical axis direction. In this case, since the collimator lens is pressed in a unidirectional direction perpendicular to the optical axis under the force of the spring means, positional displacement in the pressing direction (e.g. the perpendicular direction) is prevented. However, there are gaps in the horizontal direction, and the position of the collimator lens may therefore be changed after the adjustment.    [Patent Document 1] Japanese Examined Patent Application Publication No. H01-28381    [Patent Document 2] Japanese Examined Patent Application Publication No. H06-85233    [Patent Document 3] Japanese Laid-open Patent Application Publication No. 2006-284653    [Patent Document 4] Japanese Laid-open Patent Application Publication No. 2004-170771    [Patent Document 5] Japanese Laid-open Patent Application Publication No. 2002-131677
As described above, there are conventionally proposed technologies that prevent change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light caused by a change in temperature; however, not all the problems have been solved.
The structures of the light source apparatuses disclosed in Patent Documents 1 and 2 are directed to dealing with change in the image location, on the surface to be scanned, in the optical axis direction of the scanning light. Accordingly, neither Patent Document 1 nor Patent Document 2 takes into account a positional change in the directions perpendicular to the optical axis of the scanning light.
In the case of a synthesized light source apparatus that synthesizes two light sources, there are two scanning light beams, as described in Patent Document 4. Particularly in this case, change in the relative positional relationship in the directions perpendicular to the optical axis results in a change in the scan interval, thereby causing defects such as variation in the scanning pitch.
Even in the case where two light sources are not synthesized, if the scanning light is shifted in a direction perpendicular to the optical axis, it goes off from a position at which the optical sensor detects the printing start timing. This results in negative effects, such as inability to print due to misdetection or a decrease in light intensity due to deflection of the scanning light from the lens.
As in the case of Patent Document 5, even if positional displacement in one direction (e.g. the vertical direction) is prevented, the position of the collimator lens may be changed after adjustment since there are gaps in another direction (the horizontal direction), thereby causing the same negative effects as mentioned above. In particular, in the case of using expansion and contraction of the tube of the collimator lens in a proactive manner, as in embodiments of the present invention, the presence of gaps is undesirable since positional displacement is likely to occur.
One example of a light source having multiple light emitting devices is a surface emitting laser. In general, the collimator lens which renders parallel light emitted from the light source has larger aberration on the peripheral part compared to the central part. Large aberration tends to lead to a large beam spot. Therefore, in the case of using a surface emitting laser, the light emitting devices are disposed within the image circle (a region having an aberration of a predetermined value or less) of the collimator lens. FIG. 10 shows an example where nine light emitting devices are provided. A light source array region 70 and an image circle region 71 approximately coincide with each other and not much room is left between them. In this condition, if the position of the collimator lens is displaced in a direction perpendicular to the optical axis as described above, the image circle is displaced, whereby some light emitting devices on the periphery of the light source array region 70 undesirably go outside of the image circle.
It is expected that, in the future, a larger number of light emitting devices than in the case of FIG. 10 will be required and the light source array region therefore increases in size. In this case, a collimator lens having a large image circle is necessary. However, there is a limit to the size of the image circle, and the light source array region 70 and the image circle region 71 approximately coincide with each other and not much room is left between them, as illustrated in the example of FIG. 10. Therefore, in this case also, positional displacement of the collimator lens in a direction perpendicular to the optical axis causes positional displacement of the image circle. Accordingly some light source devices on the periphery of the light source array region 70 undesirably go outside of the image circle.